Spherical ultra high molecular weight polyethylene

ABSTRACT

A proces for preparing a spherical support for the polymerization of alpha-olefins from an ammonium dawsonite which is spray-dryed and formed into spherical particles which are then calcined and impregnated with titanium to produce a spherical catalyst of good mechanical strength is described. Also described is the polymerization process which, in the presence of the spherical catalyst, yields polyolefin particles which preserve the spherical characteristics of the support, with low flow angle and good bulk density, as well as the product polyethylene obtained from the process.

This is a Continuation of application Ser. No. 08/730,019 filed Oct. 11,1996, now abandoned, which is a Continuation of application Ser. No.08/466,051 filed Jun. 6, 1995, now abandoned, which is a Divisional ofapplication Ser. No. 08/222,916 files Apr. 5, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The present invention refers to a process for preparing a catalyst forthe polymerization of alpha-olefins under low pressure using aZiegler-Natta catalyst system, as well as to the spherical catalyst soobtained and to the process for preparing spherical polyethylene ofultra-high molecular weight in the presence of such catalyst. Morespecifically, the present invention refers to the process for preparinga spherical catalyst support, the characteristics of the support beingsuch that the spherical shape as well as the high mechanical strengthare preserved during drying, calcination and impregnation so as thecatalyst prepared from the support as well as the ultra-high molecularweight polyethylene prepared from the catalyst system preserve thesupport spherical shape, which causes better flow properties as well asother morphological properties of the polyethylene.

PRIOR ART

In using Ziegler-Natta as catalysts for producing polymers, there is acontinuous need for techniques which would lead to better processing,higher bulk density and use of lower amounts of antioxidant in theshelf, these aspects being linked to the morphology of the productpolymer.

The concern with the control of the polymer morphology has been theobject of numerous fundamental studies as well as of the practical,industrial and therefore patent able consequences which derivetherefrom.

Thus, French patent FR 2,071,111 owned by Solvay, teaches that supportedcatalysts allow for the absolute control of the polymer morphology, themorphologies of the support and the polymer being linked. This can bestated in the case where the support has the shape of a microsphere, thepolymer obtained having the shape of small spheres, as set forth inFrench patent 1,550,186. In FR 2,07,111, a metal halide of Groups IV, Vand VI of the Periodical Table in its maximum valence state is reducedon a support by means of an organic compound such as an aluminum alkyl,the support being previously impregnated with one of the reagents whichmake up the catalyst, in the liquid state while introducing theimpregnated support into the other reagent which is found either pure inthe liquid state, either dissolved in a solvent. It is alleged that acorrect kind of support for the objectives of the patent are theso-called "cenospheres" which are made up of porous spheres of diameterbetween 50 and 250 microns, each sphere being a collection of units ofdiameter between 0.2 to 2 microns. Thus, while the external shape of thecenosphere determines the morphology of the polymer produced with theaid of the supported catalyst, it could equally be seen that theelementary particles which constitute the cenosphere are regularlyspread on the polymer. These particles can act as nucleating centerswhen the polymer crystallizes. There is a comment in this reference thatdue to the fact that the polymer formed is an increased image of thesupport, the cenosphere granulometry is reflected on the granulometry ofthe polymer beads and consequently influences the bulk density of thepolymer. Normally, a high bulk density is sought which is obtained froma support of wide distribution of particle sizes, especially a bimodaldistribution, maxima being found at 55 microns and 125 microns. Themorphology and bulk density of the support are equally monitored by thechoice of the support, which makes possible to reach slurries of highdensities during polymerization while the particle size of the polymerwhich exits the polymerization vessel is such that it does not requiregranulation. As a consequence of the effect of the support, a highactivity, good morphology catalyst is produced. The described catalystsare useful in the polymerization or co-polymerization of allalpha-olefins.

A. Munoz-Escalona, in an article published in the Polymer Preprints ofthe American Chemical Society, Division of Polymer Chemistry, 24(1),112-13 (1983), states that the catalyst support, more than thepolymerization technique, controls the morphology of the polymerparticles obtained through supported Ziegler-Natta catalysts. In anotherarticle by A. Munoz-Escalona and A. Sierraalta, published by the ActaCient. Venez. 34 (3-4), p. 203-8 (1983), the authors teach that in theethylene polymerization catalyzed by Et₂ AlCl--TiCl₄ the Al/Ti ratio isthe most important factor affecting the morphology of producedpolyethylene, an increase of this ratio causing an increase in thecrystallinity and the density as well as an increase in the particlesize of the polymer. As the Al/Ti ratio increases the bulk density alsoincreases while the molecular weight is reduced. EP 252804 describescatalysts the morphology of which is preserved during Polymerization.This patent teaches that ethylene is polymerized on spherical catalystswhich contain transition metals, magnesium compounds as well halides upto an adequate degreee of polymerization, the catalyst being thentreated with the aluminum compounds to stabilize the sphericalmorphology. In EP 468070 (corresponding to Japanese patent JP 221112) inthe name of J. Kano et al., entitled "Process for Preparing SphericalSilica Gel", a method is described for preparing spherical silica gelwherein the amount of water present in the paste is adjusted to be offrom 0.2 to 1.5 times the weight of silica hydrogel, in the process forpreparing spherical silica gel during the spray drying of the paste ofsilica hydrogel and water.The silica hydrogel paste is obtained byreaction of the alkali metal silicate salt and mineral acid followed byhumid granulation of the hydrogel silica, the pH of the silica hydrogelbeing in the range of from 1 to 3. Although it is alleged that theobtained spherical silica is adequate as catalyst support, nosignificant example of the produced silica as catalyst support isprovided In Brazilian patent PI BR 8005302, of the Applicant and herebyfully incorporated as reference, is described a process for preparing analumina useful as catalyst support or as a catalyst from the reaction ofaluminum sulfate and ammonium bicarbonate at 15°-20° C., the pH beingmaintained between 7.5 and 7.7 through the addition of ammoniumhydroxide, to produce the precursor ammonium dawsonite, which containsof from 10 to 20 weight % of residual sulfate ions. The calcination ofthe ammonium dawsonite at 600°-800° C. for 4-10 hours yields an aluminaof surface area 200-400 m² /g, pore volume 1.5 to 3.5 cm³ /g and where85% of the pores are greater than 100 A. In order to avoid sulfatelosses, the precursor ammonium dawsonite is not washed prior tocalcination.

In U.S. Pat. No. 4,983,693, corresponding to Brazilian patent 8707098,of the Applicant and herein fully incorporated by reference, a catalystfor the polymerization of alpha-alefins is described which is obtainedby impregnating the alumina taught in Brazilian patent 8005302 with offrom 0.8 to 1.0 weight % of titanium from titanium halide in η-hexane,activated by triisobutyl aluminum or triethylaluminum, the molar ratioof Al/Ti in the catalyst being 15/1 up to 60/1. The thus obtainedpolyethylene has ultra-high molecular weight and is used as anengeneering plastics in view of its outstanding mechanical properties,chiefly high impact and abrasion strength as well as high tensilestrength. However, the polyethylenes obtained through such a processshow a drawback as regards their morphological properties, that is,particles are irregular and of low bulk density (0.25 to 0.30 g/cm³).Additives can be added to the polymers to increase their bulk density;however, this practice increases cost as well as impurities in thefinished product. The irregular morphology of the polymer particlesproduced according to the process of U.S. Pat. No. 4,983,693 necessarilycauses fluidity problems which reflect directly on the polymerprocessing and storage. Besides, irregular polymer particles requirehigher antioxidant amounts--nearly toxic--which severely limits its usein the food industry.

In pressing morphologically irregular polymers, defficient flow andpacking cause air bubbles in the pressed items, which then show inferiorabrasion strength.

Thus, it can be seen that, in spite of the existence of numerousacademic studies on the morphology of catalyst supports, the catalystsmade from those supports and their influence on the produced polymer,aswell as patents which suggest the use of spherical supports as beingable to convey the spherical shape to the polymer (replicationphenomenon), the scientific literature has not yet published norsuggested a process or a support for a Ziegler catalyst which would beeasily prepared by the industry in spherical shape, and which aftercalcination and impregnation keeps mechanical properties intact, chieflygood wear strength, so as to convey to the polymer the replicationphenomenon. The replication phenomenon causes that the support orcatalyst conveys to the polymer its own morphology, the product showingthen optimum characteristics as regards bulk density and molecularweight, with low requirements in anti-oxidant.

SUMMARY OF THE INVENTION

One objective of the invention is a spherical catalyst support based onammonium dawsonite which is able, by means of the replicationphenomenon, to convey its morphological properties to the catalyst aswell as to the produced polymer.

Another objective is a spherical support which is prepared by thespray-drying of the ammonium dawsonite slurry, the morphologicalproperties of which are preserved in the catalyst and fully conveyed tothe produced polymer, which will then show optimum properties asconcerns bulk density and flow, while keeping at a minimum the need ofanti-oxidant additives.

Still another objective is a spherical, easily processable polyolefinpowder,which requires low or no amounts of additive for improving bulkdensity and oxidation resistance.

These objectives are attained by using a well-knowntechnique--spray-drying--on an active support, of high surface area,which leads to a spherical product of new features, which, through thereplication phenomenon, have been conveyed to the product polymer, ofexcellent morphology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a picture taken at the microscope of a spherical polyethyleneof the invention, magnification being 8 times.

FIG. 2 is a picture of the same polymer of FIG. 1, the magnificationbeing 80 times.

FIG. 3 is a picture of a commercial polyethylene, the magnificationbeing 10 times.

FIG. 4 is a picture of the same commercial polyethylene of FIG. 3, themagnification being 500 times.

By examining the attached Figures it can be seen that the morphologiesof the polymers are completely different, this having directconsequences on the physical properties of the product polyethylenes.

PREFERRED MODE--DETAILED DESCRIPTION

In preparing the present support and catalyst, various materials can beused to synthesize the precursor ammonium dawsonite, as described inBrazilian patent BR 805302. In using the best mode to perform thepresent invention, an aqueous solution of commercial aluminum sulfate at216 g/l and an aqueous solution of commercial ammonium bicarbonate at230 g/l are made to react at 15°-20° C., the pH being controlled between7.5-7.7 by adding ammonium hydroxide so as to obtain the ammoniumdawsonite containing of from 10 to 20% by weight of residual sulfateions. In order to preserve the sulfate ions, the precursor should not bewashed. The dawsonite aqueous solution is filtered and the resultingfilter cake is resuspended in water so as to obtain a slurry theconcentration of which is between 8.0 and 10.0 weight %. This slurry isthen directed to a spray-dryer of 200 kg/hour of evaporation capacity,the liquid product being spray-dryed by means of a rotating disk. Theentrance conditions into the spray-dryer are entrance temperature offrom 350°-450° C., exit temperature 130°-150° C., disk speed between10000 and 14000 rpm, flowrate of from 3.0 to 4.0 kg/minute. Spray-dryingyields support spherical particles of mean diameter between 38 to 61microns. If particles of lower diameters are desired, either thedawsonite slurry concentration or the spray-dryer rotation speed isreduced. The dried dawsonite is placed in a quartz tube in a horizontalfurnace and heated to 600°-700° C. for 4 to 6 hours. After calcinationthe alumina is transferred to a one liter capacity vessel. Thegamma-alumina obtained after calcination shows surface area between150-250 m^(2/) g and pore volume of from 1.0 to 2.0 ml/g. On thisgamma-alumina are impregnated of from 0.5 to 1.0 weight % of titanium asthe halide in n-hexane, as described in U.S. Pat. No. 4,983,693, of theApplicant, herein fully incorporated by reference.

The low-pressure polymerization is effected following the generalprocedure outlined in Brazilian patent PI 8707098, ethylene pressurebeing of from 14 to 20 kgf/cm². In case of alpha-olefins or copolymers,the process conditions are adapted so that, for the specific kinetic andthermodynamic conditions,can be obtained the desired polyolefin orspherical copolymer.

Therefore, the process for preparing the spherical catalyst according tothe present invention comprises the following steps:

A) Spherical support

a) in a centrifuge, filter an aqueous slurry of ammonium dawsoniteprepared from an aqueous solution of aluminum sulfate and an aqueoussolution of ammonium bicarbonate at pH 7.5-7.7, and resuspend in waterthe filter cake so as to obtain a slurry of concentration between 8.0 to10.0 weight %;

b) feed the slurry of step a) in a spray-dryer at a flowrate of 3.0 to4.0 kg/minute, the entrance temperature in the spray-dryer being of from350° to 450° C. while the exit temperature is of from 130° to 150° C.;

c) dry the spherical ammonium dawsonite prepared in b) the spray-dryerrotating disk operating between 10000 to 14000 rpm;

d) calcine the spherical, dried ammonium dawsonite prepared in c) in aquartz tube placed in a furnace at 600°-700° C. for 4-6 hours, yieldinga spherical gamma-alumina of surface area of from 150-250 m² /g and porevolume of between 1.0 to 2.0 ml/g;

B) Spherical catalyst

a) impregnate the gamma-alumina of A)d) with of from 0.5 to 1.0 weight %of titanium as the halide and dissolved in n-hexane at a temperature offrom 140°160° C. which after one hour is reduced to 60°-65° C., thereaction product being washed three times with aliphatic hydrocarbon(n-hexane) and stored in this same hydrocarbon, the final titaniumcontent in the catalyst being of from 0.5 to 1.0 weight %.

For the polymerization of olefins such as ethylene, n-hexane solvent isfed to the polmerization vessel and heated to 80° C., then co-catalystAl (Et)₃ and spherical catalyst are introduced, Al/Ti ratio being ofbetween 15/1 to 60/1. When temperature reaches 85° C., ethylene is fedat a pressure of from 14 to 20 kgf/cm² for one hour or more, thenpressure of the polymerization vessel is alleviated.

The reaction yields spherical ultra-high molecular weight polyethylene.Advantageously, the polyethylene has a bulk density of 0.39 to 0.41g/cm³, an internal attrition angle of 30° to 40°, a tensile strength of300 to 440 kgf/cm², an elongation of 195 to 260%, a Rockwell hardness of61 to 67, does not flow when subjected to the ASTM D-1238 melt flowindex test and does not break when subjected to the ASTM D-256 IzodImpact strength test.

The present invention will be now illustrated by the following Examples,which should not be construed as limiting.

EXAMPLE 1

As previously described, an ammonium dawsonite was synthesized which, byworking at an entrance temperature in the spray-dryer of 400° C., exittemperature of 150° C. and slurry concentration of 8.3 weight % produceda spherical catalyst support having particles of mean diameter 40microns. After calcination, at 700° C./5 hours, on this support wasimpregnated a metal titanium content of 0.55 weight %, as taught inBrazilian patent BR 8707098 (U.S. Pat. No. 4,983,693). Ethylene (14kgf/cm²) was palsmerized in a pilot plant in the presence of thiscatalyst and Al(Et)₃ as co-catalyst, the ratio Al/Ti being 40.2, thepolyethylene so obtained having spherical particles of mean diameter 650microns. The catalytic activity reached 139,037 grams of polymer pergram of titanium per hour, the bulk density of the polymer so producedwas 0.39 g/cm³ and in the flow test the internal attrition angle reached40°. Note that the internal attrition angle is a property of the productpolyolefin linked to the morphology, the attrition angle decreasing asthe powder flow increases. The definition of the attrition angle can befound in the publication by ZENS, F.A. & OTHMER, D.F. --"Fluidizationand Fluid Particle Systems", New York Reinhold Publishing Corporation1960 p. 75.

Table 1 below lists properties of the catalysts, besides polymerizationdata as well as of the product polymer for Examples 1, 2, 3 and 4.

TABLE 1

Data from Table 1 show the high catalytic activity of the sphericalsystem, comparable to high performance, non-spherical systems, as wellas the high bulk density and low internal attrition angle, whichindicate good fluidity and flow of the product polymer

Table 2 below lists physical chemical data as well physical propertiesof the polyethylenes prepared according to the present invention. Forthe sake of comparison are also listed the corresponding properties fornon-spherical polyethylenes of U.S. Pat. No. 4,983,693.

Table 2 shows that the good mechanical properties of the polyethylenesof U.S. Pat. No. 4,983,693 have been preserved through the presentprocess, while bulk density and internal attrition angle have beenimproved, which greatly favors processing.

                  TABLE 1                                                         ______________________________________                                                                        PE                                                   Support    Ti            Mean                                                 Mean Diameter                                                                            cont.   Al/T1 Diameter                                                                             Cat. Activity                          Ex. no.                                                                              (microns)  (%)     ratio (microns)                                                                            gPE/gTi h                              ______________________________________                                        1      40         0.55    40.2  650    139.037                                2      40         0.55    38.0  530    102.930                                3      40         0.55    30.0  680    238.461                                4      40         0.69    28.0  650    185.000                                Non    30         0.80    20.0  300    150.600                                Spher.                                                                        ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________       Actual             Rockwell                                                   Density                                                                            Tensile                                                                              Elongation                                                                           Hardness                                                                            bulk  Internal                                       ASTM D-                                                                            Strength                                                                             ASTM-D-638                                                                           ASTM-785                                                                            Density                                                                             Attrition                                   EX 1601 ASTM-D-638                                                                           (%)    (R)   ASTM-1895                                                                           Angle                                       __________________________________________________________________________    EX. 1                                                                            0.9357                                                                             400-440                                                                              220-250                                                                              64-66 0.39  40°                                  EX. 2                                                                            0.9352                                                                             350-400                                                                              195-240                                                                              65-67 0.39  40°                                  EX. 3                                                                            0.9370                                                                             330-420                                                                              220-260                                                                              65-67 0.41  30°                                  EX. 4                                                                            0.9369                                                                             300-400                                                                              200-230                                                                              60-62 0.41  30°                                  Non                                                                              0.9350                                                                             370-490                                                                              250-330                                                                              70-75 0.32  60°                                  spher                                                                         __________________________________________________________________________

Notes

1) In ASTM Method D-1238: Melt Flow Index in 9/10 minutes, the polymerdoes not flow, which means molecular weight higher than 4.5 millions.

2) In ASTM Method D-256, Izod impact strength in kg cm/cm, the polymerdoes not break, due to the extremely high molecular weight exhibited bythe polymer.

We claim:
 1. A spherical ultra high molecular weight polyethylene havinga bulk density from 0.39 to 0.41 g/cm³, internal attrition angle from 30to 40, tensile strength from 300 to 440 kgf/cm², elongation between 195and 260%, and Rockwell hardness from 61 to 67, wherein the polyethylenedoes not flow in the MFI test and does not break in the Izod impacttest, wherein the polyethylene has a mean diameter of at least 530microns, andwherein the polyethylene is prepared by a process forpreparing a spherical polyethylene of ultra high molecular weight usinga Ziegler-Natta catalyst system in a hydrocarbon solvent which comprisescontacting ethylene monomer with the catalyst system in a hydrocarbonsolvent and carrying out polymerization for one to three hours in thepresence of such catalyst system at 70°-85° C. and an ethylene pressurebetween 14 to 20 kgf/cm², wherein the catalyst system is prepared by aprocess which comprises the following steps:a) spray-drying an aqueousslurry at 8-10 weight % of ammonium dawsonite, the dawsonite beingsynthesized through the reaction of aluminum sulfate and ammoniumbicarbonate at a pH from 7.5 to 7.7, wherein the ammonium dawsonite isfiltered only between the reaction and the spray-drying, the entrancetemperature in the spray-dryer being from 350° to 450° C. and the exittemperature being from 130° to 150° C., the feed flowrate of theammonium dawsonite slurry being from 3.0 to 4.0 kg/minute, and the diskspeed being from 10000 to 14000 rpm, and calcining the product from thespray-dryer at 600°-700° C. for 4 to 6 hours so as to obtain a sphericalgamma-alumina of pore volume from 1.0 to 2.0 ml/g and surface area from150 to 250 m² /g while the residual sulfate content is between 10 and 20weight %; b) impregnating the alumina from a) with a titanium halidesolution in a hydrocarbon solvent at 80°-140° C. during one hour or moreso that the final titanium content incorporated is from 0.5 to 1.0weight %, thus making a catalyst composition; and c) contacting thecatalyst composition from b) with an alkyl aluminum co-catalyst so as toprovide an Al/Ti ratio from 15/1 to 60/1.
 2. A spherical ultra highmolecular weight polyethylene as in claim 1, wherein the internalattrition angle is 30°.
 3. A spherical ultra high molecular weightpolyethylene as in claim 1, wherein the polyethylene has a mean diameterof from 530 microns to 680 microns.
 4. A spherical ultra high molecularweight polyethylene as in claim 2, wherein the polyethylene has a meandiameter of from 530 microns to 680 microns.